Recently, as the integration level of semiconductor devices has increased, it has been estimated that the metal oxide semiconductor (MOS) structure that was used, until now, for basic switching devices, has reached its useful limit. In one example case of using the MOS structure, a gap between a source and a drain will become narrow in highly integrated devices such as 4 gigabyte (GB) dynamic random access memory (DRAM), such that the switching function performed by the MOS gate voltage, which is a principle for driving the MOS device, will not work. That is, at high levels of integration, even when the gate voltage is not applied to a MOS device, tunneling effects will occur between the MOS source and drain and through a gate oxide layer, resulting in malfunctioning of the MOS device. For this reason, the integration limit of the MOS structure will be about 4 GB DRAM. Accordingly, to fabricate devices in the gigabyte range and to integrate devices in the even denser terabyte range, another structure rather than the conventional MOS structure should be adopted. One of the device structures identified as an alternative to the MOS structure is a Single Electron Transistor (SET), which has been proposed by many research groups.
The SET is device that operates using a so called Coulomb blockade effect for controlling flows of individual electrons by suppressing the tunneling of electric charges while charge carriers such as electrons or holes passing a dielectric layer among many quantum mechanic phenomena caused by interactions of the electrons in nanoscale.
The Coulomb blockade effect of the single electron tunneling can be explained as follows. When the entire capacitance is very low by a region to which the electrons input through the tunneling effect, it is possible to observe the discrete electric charging of the electrons. If the energy e2/2c charged by the discrete electric charging is greater than an energy (kBT) generated by the thermal vibration and there is no voltage increase applied from outside, it is difficult to obtain the energy for charging the capacitor with electrons through the tunneling at that temperature. Accordingly, the electron tunneling does not proceed after a single electron is charged. That is, the previous tunneled electron charges the capacitor such that the next electron is given the low voltage as much as a voltage drop at the capacitor, whereby the energy for charging the capacitor through the tunneling become insufficient, which has the result of stopping the tunneling. Such an effect in which a previously tunneled electron blocks subsequent tunneling is called the Coulomb blockade effect.
Typically, the SET structure using the Coulomb blockade effect provides a channel including a conductive quantum dot, which enables the discrete flow of electrons with the identical source, drain, and gate structures. Accordingly, the channel consists of dielectric material and quantum dots so as to enable the electron to flow through the quantum dot by the discrete tunneling.
To fabricate a nanoscale device using quantum dots, it is required to develop techniques capable of forming a fine and uniform quantum dot having single crystalline characteristic. In the art of forming the quantum dot, typically, methods for forming the quantum dot include typical lithography techniques and include depositing the silicon oxide (SiO2) under excess condition of the silicon level.
The U.S. Pat. Nos. 6,597,036 and 6,060,743 have disclosed a method for forming a gate of a transistor using the quantum dots and a method for forming the quantum dot by moving a reactive atom using a beam or core formation method, respectively.